I think we should first define what evil is. Ther are so many ways to go about this really...
There are so many ways to define it but we have to be careful to not define it with our own prejudices and opinions. Example, for most people cannibalism would be evil. An isolated tribe who eats their dead would disagree. They may consider it an honour to be eaten after their death as it gives life back to the tribe. Members of one religion will believe those of another religion to be evil. Anything someone doesn't like or understand, they will often call evil. The silliest example of this was when I was called evil because I like pvp in certain games.
I mean for Astarion, exterminating the goblins is not morally bad, because goblins are vermin, and I agree with him. Killing goblins is not an evil thing to do. So we can establish that simply killing a bunch of deminhumans is not neccessarily evil. So then what is evil?
You can establish. I would disagree. I would kill them (or anyone) if they were hostile to defend myself or those I care about, but if there was an option to not do so then I would take it. I do not kill based on race but rather on actions, even my Drow characters (who I tend to play as evil) would consider some of other races more useful alive.
I think evil is malevolence. When you not only do the deed but derive a significant amount of satisfaction from performing it to the point that it becomes an incentive for you to keep doing it. This leads to something of a psychopathic character, but I don't think most evil people are well adjusted at all anyway.
This may often be the case but not always. Again, this is opinion. I have met people who I would consider evil and they were very well adjusted but perhaps missing something in the brain. Consider also that many Drow are evil because they were raised to be this way and it is all they know. They do not all do what they do for satisfaction but rather for survival.
If you give a bunch of benefits to the evil path to compel people to walk it, you are suddenly turning evil into reasonable, in fact your character becomes more reasonable then evil.
Why would this be an issue? Not all evil people do what they do because they want the evil label. If being reasonable accomplishes the desired goal then this should be fine.
Situation: I'm infected with a disease that can kill me at anytime The druids direct me to find an archdruid who may or may not be able to cure me The goblins tell me they will give me a surefire way to keep control over the disease is I do their bidding.
It's completely reasonable, and thus morally defensible to side with the goblins in such a case (this is not what happens in BG3, its just an example)
This could have been an interesting path.
Evil is also something that stems from the inside. In the situation above, we don't yet know whether you are sorry to work with the goblins but are strung along due to your circumstances or whether you revel in killing innocents in their name. This must be exposed through dialogue and that's where companions should come in and comment on your deeds, so that you can respond in different ways to express your true intentions and motivations.
Evil can also be created from outside influence, not just from inside. Companions should have more dialogue options related to all large choices, being able to explain our reasoning to them would be wonderful.
An evil character would always say something along the lines of: "I don't care if others got hurt, because I got my way" or "I enjoyed harming them, it caused me great joy".
Not always, but these are the stereotypical types. There are different shades of evil. Some may even regret that they had to harm others.